1. Field of the Invention
The present invention relates to a boron carbide sintered compact having a light weight and good mechanical properties, and to a protective member used for a protector that reduce the penetration power of a projectile, such as a bullet or an artillery shell, to protect human bodies, vehicles, vessels, and aircraft.
2. Description of the Related Art
With recent tense international situations, the demand for protective members has been increasing. These protective members require a light weight, and in addition, a high compressive strength because a high compressive stress is applied from a bullet, an artillery shell, or the like to the protective members. An example of such a material having a light weight and excellent mechanical properties is a boron carbide sintered compact, which has been practically used as a protective member against a bullet, an artillery shell, or the like.
An example of a boron carbide sintered compact is boron carbide described in Japanese Patent Application No. 58-80257 (Japanese Unexamined Patent Application Publication No. 58-204873). According to this patent document, a fine granular mixture containing α-silicon carbide, boron carbide, and carbon and/or an organic substance that cokes is sintered without applying pressure. The resulting sintered compact is then compressed again by hot isostatic pressing (HIP) in a high-pressure autoclave using an inert gas as a pressure-transmitting medium to produce a boron carbide sintered compact.
However, this boron carbide sintered compact is produced by a complex process including sintering a fine granular mixture containing α-silicon carbide, boron carbide, and carbon and/or an organic substance that cokes without applying pressure, and compressing again the resulting sintered compact by hot isostatic pressing (HIP) in a high-pressure autoclave using an inert gas as a pressure-transmitting medium. Accordingly, the sintered compact cannot be produced at low cost. Furthermore, when carbon is used, both graphitizable carbon having a layered crystal structure and non-graphitizable carbon having a crystal structure with a cross-linked lattice are present. As a result, fine pores and amorphous portions are easily formed inside coke, resulting in a problem of unstable mechanical properties of the sintered compact. When an organic substance that cokes is selected, the variation in the quality of coke is significant, and crystallinity of the coke is also low. Accordingly, as in the case where carbon is selected, satisfactory mechanical properties, such as compressive strength, cannot be stably obtained.
According to Japanese Unexamined Patent Application Publication No. 7-41365, a mixture prepared by mixing a boron carbide powder and a component composed of a preceramic organic silicon polymer selected from the group consisting of polysiloxanes, polysilazanes, polysilanes, metallopolysiloxanes, and metallopolysilanes is molded under pressure at about 500° C. or lower so as to have a desired shape. The compact is then sintered in an inert gas atmosphere at about 2,200° C. or higher to produce a boron carbide sintered compact.
However, since charcoal is produced from the preceramic organic silicon polymer selected from the group consisting of polysiloxanes, polysilazanes, polysilanes, metallopolysiloxanes, and metallopolysilanes, the variation in quality of the charcoal is significant, and crystallinity of the charcoal is also low. Accordingly, satisfactory mechanical properties cannot be stably obtained.
According to Japanese Unexamined Patent Application Publication No. 2003-201178, a mixture of a solvent and a boron carbide powder coated with a polymeric organic substance that is not substantially dissolved in the solvent is molded under pressure to prepare a compact. The compact is then sintered to produce a boron carbide sintered compact which contains graphite crystal grains in an amount in the range of 1 to 5 parts by weight and in which these graphite crystal grains are mainly present on triple points of the boron carbide crystal grains.
The presence of the graphite crystal grains mainly disposed on the triple points of the boron carbide crystal grains accelerates densification, thereby improving mechanical properties to some degree. However, for the application to protective members, only the graphite crystal grains cannot strongly bind boron carbide grains, and a satisfactory compressive strength cannot be provided.
Other four examples of a boron carbide sintered compact will now be described.
First, Japanese Unexamined Patent Application Publication No. 6-87654 describes a boron carbide sintered compact produced by mixing 15 to 40 volume percent of graphite, 10 volume percent or more of a boron carbide powder X having an average particle diameter of 7 μm or 12 μm, and 30 volume percent or more of another boron carbide powder Y having an average particle diameter of 12 μm or 30 μm to prepare a base powder; filling a die with the base powder; molding the base powder at a temperature in the range of 480° C. to 600° C. under pressure to prepare a compact; and sintering the compact at 2,150° C. under normal pressure. According to the description, only the average particle diameters of the boron carbide powders X and Y are limited so that the average particle diameter of the boron carbide powder X is ½ or less of the average particle diameter of the boron carbide powder Y.
Secondly, Japanese Unexamined Patent Application Publication No. 2003-137655 describes a boron carbide sintered compact containing 99.5 to 70 mole percent of boron carbide (B4C) and 0.5 to 30 mole percent of aluminum nitride (AlN) with a relative density of 95% or higher. As described in Example 4 of the patent document, this boron carbide sintered compact is produced by, for example, the following method. Ten mole percent of an aluminum nitride (AlN) powder is added to a boron carbide powder having an average particle diameter of 0.4 μm and the maximum particle diameter of 2.3 μm, and a methanol solvent, which does not readily oxidize the aluminum nitride powder, is added to the mixture to prepare a mixed powder. The mixed powder is molded, and the compact is then sintered while nitrogen gas is continuously supplied to a sintering furnace at a flow rate of 0.006 liter/min or more so that the nitrogen partial pressure in the sintering furnace is 3.1×10−4 MPa or higher.
In the boron carbide sintered compact produced as described above, the aluminum nitride is easily decomposed during sintering. Consequently, a dense boron carbide sintered compact cannot be stably produced, and therefore, the sintered compact does not have a satisfactory compressive strength.
Thirdly, Japanese Unexamined Patent Application Publication No. 2004-26633 discloses a boron carbide sintered compact containing boron carbide (B4C) and 10 to 25 mole percent of chromium diboride (CrB2) and having a relative density of 90% or higher. In this boron carbide sintered compact, the maximum grain size of boron carbide grains is 100 μm or less, and the ratio (area ratio) of boron carbide grains having a grain size in the range of 10 to 100 μm to boron carbide grains having a grain size of 5 μm or less is in the range of 0.02 to 0.6.
In this boron carbide sintered compact, chromium diboride, which cracks easily when a compressive stress is applied thereto, is present in the grain boundaries. Therefore, in particular, the sintered compact does not have a satisfactory compressive strength.
Finally, Japanese Unexamined Patent Application Publication No. 2002-167278 discloses a protective member for reducing an impact by collision of a projectile, including a boron carbide sintered compact produced by molding a boron carbide powder having an average particle diameter in the range of 0.3 to 1.5 μm by slip casting and then sintering the resulting compact. The boron carbide sintered compact is produced by the following method.
More specifically, a predetermined amount of novolak phenolic resin (specific gravity: 1.18, manufactured by Showa Highpolymer Co., Ltd.) is added to a boron carbide powder (manufactured by Electroschmelzwerk Kempten GmbH) having an average particle diameter of 0.74 μm and a specific gravity of 2.5, and the phenolic resin and the boron carbide powder are mixed in an acetone solution. The acetone is then completely evaporated, and the mixture is then pulverized, thus preparing a boron carbide powder coated with the phenolic resin. The boron carbide powder is mixed with water under stirring so that the content of the boron carbide powder is 25 volume percent and the water content is 75 volume percent. The mixture is further mixed and deaerated in vacuum to prepare a slurry for slip casting. The slurry is poured into a gypsum mold to perform drain casting, and is then sintered in Ar gas at a temperature in the range of 1,200° C. to 2,250° C. for 1 hour 40 minutes and then at 2,250° C. for 30 minutes. However, from the standpoint of mass production, the boron carbide sintered compact produced by this method is not suitable for forming a small and simple shape, for example, a cylindrical shape, an annular shape, or a spherical shape, which is used for protective members. Furthermore, the boron carbide sintered compact does not have a satisfactory compressive strength.